Theoretical approaches are being developed to extract information concerning the nature of internal motions in a variety of biopolymers from nuclear magnetic relaxation and time-resolved fluorescence depolarization experiments. The recently developed model-free approach to the analysis of NMR relaxation has made possible a new and stringent test of molecular dynamics simulations. By comparing order parameters extracted from an NMR study of pancreatic trypsin inhibitor with those calculated from a simulation, it was shown that while theory predicts less motion, the relative flexibility is well described. The role of diffusion in reaction dynamics has been investigated. A new theory has been formulated to describe situations where reactivity fluctuates in time (e.g. the accessibility of a binding site is modulated by a gate). Monte Carlo and finite-difference algorithms have been developed for the numerical solution of complicated diffusion problems.